Technical Field
The present invention relates to the field of wireless data transmission, and in particular to wireless data transmission by modulation based on a carrier frequency signal.
Discussion of the Related Art
FIG. 1 illustrates a wireless data transmission system 100 comprising a transmission side 102 and a reception side 104. This topology is known as a homodyne or direct conversion technique. On the transmission side 102, a local oscillator 106 (LO), usually based on a phase locked loop (PLL) 108, generates a frequency signal fLO. The frequency signal fLO is provided to a data modulator 110 (IQ MOD), which receives a data signal comprising I (in phase) and Q (in quadrature) components. Data modulator 110 modulates the data signal based on the frequency signal fLO. The output modulated signal S(t) is then transmitted via an antenna 112.
On the receive side 104, a receive antenna 114 receives the modulated data signal S′(t), and provides it to a data demodulator 116 (IQ DEMOD). Demodulator 116 demodulates the data signal based on a frequency signal f′LO provided by local oscillator 118, which mainly comprises a carrier recovery block. The purpose of the carrier recovery block is to synchronize the frequency signal f′LO with the carrier frequency of the signal S′(t) in both frequency and phase. For this, a first few data blocks of the data signals I′ and Q′, estimated based on the receive signal S′(t), are provided to an analog-to-digital converter 120 (ADC), which generates digitalized data signals I′ and Q′, in turn provided to a digital processing block 122. Block 122 for example corresponds to a Costa loop, and outputs a correction signal on a line 124, which is proportional to the frequency/phase difference between the signals f′LO and S′(t), in a similar manner to a frequency/phase locked loop. The first few blocks of the I′ and Q′ data cannot generally be correctly demodulated and only form a preamble used for synchronization.
A problem with the transmission system 100 of FIG. 1 is that a local oscillator is needed on both the transmission and the reception sides, and in order to ensure successful data transmission, the frequency signals fLO and f′LO have to be well synchronized. At relatively high frequencies this is difficult, as the phase noise and frequency instability of the frequency signal fLO, increase by a squared relation to its frequency. Furthermore, the phase locked loop 108 on the transmission side, and the ADC 120 and digital decoding circuitry 122 on the reception side add complexity, energy consumption and cost to the transmission system.